Determination of the Rotation Curve of the Milky Way Using the 21 cm Hi Emission Line

Abstract

In this paper, the rotation curve of the Milky Way galaxy has been determined using the observed HI emission line at a wavelength of 21 cm. Particularly, the Tangent Point Method was used in order to measure the rotational velocity and the distance to the center of the Milky Way. The measured rotation curve showed that the rotational velocity remains approximately constant at large distances from the center of the Galaxy. This is actually an evidence for the existence of dark matter in the halo of the Milky Way. If all the matter in the Milky Way is visible, then the behavior of the rotation curve of the galaxy should experience a Keplerian decline. The mass of the Milky Way within a radius of 15 kpc was also estimated to be ~ 1.65 × 10 11 M⊙ which represents the mass of luminous matter in the Galaxy. However, if one assumes that the dark matter halo extends to 50 kpc, then the mass of the Galaxy should be ~ 5. ISSN: 0067-2904 Mahdi and MohsinIraqi Journal of Science, 2017, Vol. 58, No.2C, pp: 1169 INTRODUCTIONThe fraction of visible matter in the Universe is only ~ 5 % of the total mass and the rest is in the form of dark matter (27%) and dark energy (68%). Therefore, it is very important to study the observational evidences of the existence of dark matter. The fraction of each component in the Universe according to PLANCK 2015 constraints [1] is shown in Figure-1. There are many evidences for the existence of dark matter in the Universe.The first evidence of the existence of dark matter was published by Zwicky in the 1930s [2,3]. He estimated the velocity dispersion of galaxies in Coma cluster and found that the velocity dispersion is higher than expected from the visible matter only. Gravitational lensing provides another evidence for the presence of dark matter in bullet clusters such as 1E0657-558. These systems comprise of two colliding clusters of galaxies and as a result of the collision, the smaller cluster passes through the main cluster. Gravitational lensing analyses show that the peak of mass distribution is offset from that obtained from x-ray analyses which represent the peak of the visible mass distribution. This also proves that the dark matter in these clusters is collisionless [4][5][6].In addition, the flatness of the rotation curves of spiral galaxies at large radial distance is an evidence for the existence of dark matter in those galaxies [7][8][9]. According to the Newtonian mechanics, there should be a balance between the centrifugal force and the gravitational force [10]: